1. Field of the Invention
The present invention relates to an improvement in an exhaust system and to the layout of a muffler for a motorcycle.
2. Description of Background Art
There have been known motorcycles to which a contrivance is applied to the layout of a muffler for various purposes (see, for example, Japanese Patent Laid-open No. Hei 4-266579 (FIG. 5)).
As shown in FIG. 5 of Japanese Patent Laid-open No. Hei 4-266579, a motorcycle (1) (the parenthesized numeral represents a reference symbol used in Japanese Patent Laid-open No. Hei 4-266579, here and below) has a configuration in which an engine (19) is mounted to a main frame (3), an exhaust pipe (46) is extended rearwardly downward from a cylinder head (27) of the engine (19), and a muffler (47) disposed on the lower side of a rear portion of the main frame (3) and a swing arm (15) is connected to a rear end portion of the exhaust pipe (46).
The muffler (47) is disposed on the lower side of the rear portion of the main frame (3) and the swing arm (15), and such a muffler layout is applicable, on a structural basis, to an off road motorcycle capable of traveling off-road. In an off road motorcycle, however, a sufficient vehicle height should be secured and, therefore, it is impossible to dispose a muffler at a lower portion of the vehicle body. Therefore, as described in Japanese Patent Laid-open No. Hei 9-254853, it has only been possible to dispose an exhaust pipe so as to protrude to the vehicle front side and to dispose a muffler on a vehicle lateral side. Such a configuration has problems in external appearance, an increase in vehicle width, etc.
When a muffler is disposed on a vehicle rear side or lateral side, the rear end of the muffler would protrudes to the vehicle rear side or a side portion of the muffler would protrude to a vehicle lateral side, leading to an increase in the size of the off road motorcycle. In addition, the external appearance may also be damaged.
There has been proposed an exhaust system for a motorcycle in which an exhaust pipe extending from an internal combustion engine is divided into a left part and a right part, and mufflers are provided respectively at tips of the left and right parts (see, for example, Japanese Patent Laid-open No. 2005-313671 (FIG. 3)).
As shown in FIG. 3 of Japanese Patent Laid-open No. 2005-313671, the exhaust system for a motorcycle has a configuration in which to a rear end of an exhaust pipe (50a) (the parenthesized numeral indicates a reference symbol used in Japanese Patent Laid-open No. 2005-313671, here and below) extending from an internal combustion engine toward the vehicle rear side, a branching section (55) is connected. From one part of the branching section (55) a first branch pipe (51) extends rearward, and a first muffler (31) is connected to the first branch pipe (51). From the other part of the branching section (55) a second branch pipe (52) extends toward a vehicle lateral side opposite to the side of the first branch pipe (51) through the rear side of a rear cushion (33), and a second muffler (32) is connected to the second branch pipe (52).
When further enhancement of silencing performance is demanded, it is necessary to take such a measure as increasing the muffler capacity of the mufflers (31, 32). An increase in the muffler capacity can be achieved by an increase in the diameter or an increase in the length of the mufflers (31, 32). When the length is increased, however, the mufflers (31, 32) may bulge to the vehicle rear side or may cause an increase in the vehicle width.
Restriction on vehicle length is greater in motorcycles than in four-wheel vehicles. Therefore, if the mufflers (31, 32) bulge to the vehicle rear side, the external appearance of the motorcycle may be damaged.
In addition, in motorcycles, mass concentration on the longitudinal center of the vehicle is desired. In this connection, bulging of the mufflers (31, 32) to the vehicle rear side makes it impossible or very difficult to achieve mass concentration.
Thus, there is a need for an exhaust system for a motorcycle that has a favorable external appearance and promises easy realization of mass concentration. Further, there is a need for a motorcycle in which a muffler can be installed in a smaller-sized form and the external appearance of a vehicle rear portion is enhanced.